This application requests supplemental funding for the Mother / Infant Study of Prevention of Milk-Borne Transmission of HIV in Botswana (R01 HD37793), a randomized, partially placebo-controlled trial being conducted in Botswana that is testing two interventions designed to prevent mother to child transmission (MTCT) of HIV. The primary objectives of the Mother / Infant ("Mashi") study are: i) To determine whether single-dose maternal and infant nevirapine (NVP, per HIVNET 012 protocol) prevents additional MTCT when added to short-course zidovudine (ZDV, starting at 34 weeks and intrapartum, and for one month to the infant), compared with ZDV alone. ii) To determine whether six months of prophylactic infant ZDV given to breastfeeding infants of HIV-infected mothers compares favorably to formula feeding of infants for six months with regard to MTCT and HIV-free survival among infants randomized to these two different feeding strategies. These important primary objectives have not changed and can be answered (with additional funding) by the Mashi study, which has been preceding very well, with good accrual, minimal loss to follow-up, and excellent data quality. However, the study has undergone several necessary modifications since its inception in order to remain ethical, in response to advances in knowledge of MTCT prevention and in local standards of medical care. One of these changes was the prolongation and intensification of maternal follow-up after findings from Kenya showed that HIV-infected women randomized to breastfeeding suffered three-fold higher mortality than those randomized to formula feeding. The other change is the provision by study staff of combination, potent antiretroviral therapy (ART) to women with AIDS and to all HIV-infected infants in the cohort, to remain in accordance with the new national standard of care in Botswana, which includes ART. These unavoidable and unanticipated changes have resulted in significant increases in the cost of study conduct, primarily due to the additional human resources needed to execute it. [unreadable] [unreadable]